Posts Tagged ‘coal fired boiler’

We’ve been discussing various aspects of a power plant’s water-to-steam cycle, from machinery specifics to identifying inefficiencies, and today we’ll do more of the same by introducing the condenser hot well and discussing its importance as a key contributor to the conservation of energy, specifically heat energy. Let’s start by returning our attention to the steam inside the condenser vessel.

Last week we traced the path of the condenser’s tubes and learned that the cool water contained within them serve to regulate the steam’s temperature surrounding them so that temperatures don’t rise dangerously high. To fully understand the important result of this dynamic we have to revisit the concept of latent heat energy explored in a previous article. More specifically, how this energy factors into the transformation of water into steam and vice versa.

Steam entering the condenser from the steam turbine contains latent heat energy that was added earlier in the water/steam cycle by the boiler. This steam enters the condenser just above the boiling point of water, and it will give up all of its latent heat energy due to its attraction to the cool water inside the condenser tubes. This initiates the process of condensation, and water droplets form on the exterior surfaces of the tubes.

The water droplets fall like rain from the tube surfaces into the hot well situated at the bottom of the condenser. This hot well is essentially a large basin that serves as a collection point for the condensed water, otherwise known as condensate.

It’s important to collect the condensate in the hot well and not just empty it back into the lake, because condensate is water that has already undergone the process of purification. It’s been made to pass through a water treatment plant prior to being put to use in the boiler, and that purified water took both time and energy to create. The purified condensate also contains a lot of sensible heat energy which was added by the boiler to raise the water temperature to boiling point, as we learned in another previous article. This heat energy was produced by the burning of expensive fuels, such as coal, oil, or natural gas.

So it’s clear that the condensate collecting in the hot well has already had a lot of energy put into it, energy we don’t want to lose, and that’s why its an integral part of the water-to-steam setup. It acts as a reservoir, and the drain in its bottom allows the condensate to flow from the condenser, then follow a path to the boiler, where it will be recycled and put to renewed use within the power plant.

Next week we’ll follow that path to see how the condensate’s residual heat energy is put to good use.

Perhaps you went out on a drive to enjoy a nice summer day. As you ventured into uncharted territory, you might have ended up in an industrial area. There, you noticed factories, chemical plants, and oil refinery complexes, each surrounded by a huge system of pipes and tanks. You might have considered it to be an eyesore, but if you’re an artist and engineer like I am, you might look at it as a form of art, composed of interesting shapes, colors, and patterns. No matter how you look at it, you can bet that there are at least a few pressurized containers in there.

Last time we saw how something as seemingly harmless as a home water heater could become a dangerous missile if the pressure inside builds to the point where the tank ruptures. You can imagine what kind of explosive forces, steam, and chemicals would be unleashed into the surroundings if an industrial sized pressurized container failed due to overpressure. Let’s explore some other types of overpressure devices that are commonly used in industrial settings.

One type of overpressure device is a safety valve. They are similar to a water heater relief valve, but they are generally used to relieve overpressure of gases and steam. How do they work? Basically, a safety valve is attached to the top of a pressurized container as shown in the cut away view in Figure 1 below.

Figure 1 – A Basic Safety Valve In The Closed Position

A powerful spring in the valve body is designed to force down on the valve and keep it closed if there is normal pressure inside the container. Once the pressure begins to rise to an unsafe level, it pushes up against the valve and overcomes the force of the spring. The valve opens, as shown in Figure 2 below, and the contents of the pressurized container are safely vented out to an area that is normally unoccupied by people. In case you’re wondering, safety valves are commonly used on pressurized storage tanks and boilers.

Figure 2 – A Basic Safety Valve In The Open Position

Another way to address the overpressure scenario is to employ a rupture disc. This is in fact a purposely constructed weak spot. It is intentionally built into a pressurized container and is designed so that it will fail when pressure starts to rise. In fact, this disc is designed to fail at a pressure point just below the pressure at which the container itself would fail. The disc is usually located within a vent pipe, which is in turn connected to the container. Should the disc rupture in an overpressure situation, the contents of the pressurized container will safely flow out of the vent pipe to a place normally unoccupied by people. The advantage of using a rupture disc is that they are made to safely release huge quantities of pressurized substances very quickly. The disadvantage in their usage is that they’re a one-time fix. That is, unlike relief or safety valves which may perform their function a multitude of times, a rupture disc is destroyed once it does its job. They are generally used in industrial settings where potential hazards are greater than at home, so once the rupture disc blows, the complete system generally undergoes a shut down so that the disc an be replaced before the pressurized container can be used again.

Another option to pressure containment is the use of a fusible plug, usually constructed of a metal that will melt if the temperature within a pressurized container rises above a certain level. The metal plug melts, and excess pressure is vented through the aperture formed into a safe location. These are often used on locomotive boilers and compressed gas cylinders. Like rupture discs, fusible plugs are a one-time fix and must be replaced once they have done their job.

Yet another option to pressure containment is to use a temperature limiting control. This category includes devices that monitor temperature and pressure within a pressurized container. If a dangerous situation should develop, the control system reacts, effectively reducing the pressure to prevent failure of the vessel. Automatic combustion control systems for boilers in electric utility power plants use temperature and pressure sensors to keep pressures within safe limits by regulating fuel and air input to the boiler.

Next time we’ll cover the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC), which establishes rules governing the design, fabrication, testing, inspection, and repair of boilers and other pressurized containers.

Are you familiar with the adage, “Things are not always as they seem”? It’s probably come into play in your life at one time or another, like when you opted to buy the cheapest model of something, only to find out that its life span was two weeks before falling apart. Not such a bargain after all.

Well, it’s kind of that way with low sulfur coal and its application in electric power production. All coals contain some sulfur, their content ranging from trace amounts to as high as 8%. This sulfur ends up as a byproduct of the combustion process, meaning it is released into the atmosphere when coal is burned. There it combines with moisture in the air to form sulfuric acid. If you will remember from last week’s blog, this is the stuff that forms acid rain, able to dissolve marble statues, corrode metal, and disrupt eco systems.

In the process of generating electricity for homes and businesses, many utility power plants of the past burned coal with high sulfur content. This was the case through the middle of the 20th Century. This coal was brought into power plants by trains and river barges from nearby coal mines. In some cases power plants were actually built next to the mines, thereby eliminating shipping cost. It was effective and cheap.

Then, in 1963, the Clean Air Act was signed into law, its purpose to improve, strengthen, and accelerate programs for the prevention of air pollution. By 1970 the Act had empowered the federal government to set and enforce national air quality standards for sources of air pollution, like coal burning power plants. Under the Clean Air Act, government was able to mandate to utilities that they reduce sulfur emissions or face court injunctions to shut them down. Caught between a rock and a hard place, utilities learned to comply, switching over to lower sulfur coals. But the story doesn’t end here. That lower sulfur created a whole host of new problems, for the power plant and their consumers.

To begin with, low sulfur coals are scarce in areas of the country where electricity is needed most, like the densely populated eastern half of the country. It has to come from mines in the western states like Wyoming, and for a power plant located in Chicago, for example, this can get costly. A lot more costly than simply getting the coal, high sulfur content coal, that is, from nearby mines in southern Illinois. The result is higher transportation costs, and this cost is passed on to consumers.

Another problem with low sulfur coals is that they tend to release less heat energy than higher sulfur coals when they are burned. That means that you have to burn more of it to generate the same amount of power. As a result, utilities ended up having to buy more coal, another cost that was passed on to the consumer.

Yet another issue with the switch from high sulfur to low sulfur coals involved the reconfiguration of power plants that was made necessary. You see, when power plant boilers are designed, they have a particular type of coal in mind, and that originally was high sulfur coal. In addition, many power plants have been required to install equipment to scrub sulfur from the gases produced when the coal is burned. This scrubbing equipment is expensive to purchase, install, and operate. Pollution control equipment like this consumes power, but it does not facilitate the process of generating electricity.

In addition to these costs, the switch to low sulfur coal causes many other problems that can raise the cost of operations and make the power plant less reliable. For example, some low sulfur coals have properties that tend to make ash stick to the surfaces inside of boilers, often leading to boilers overheating and springing leaks. If these leaks are bad enough, the boiler has to be shut down for cleaning and repair, and when this happens the electrical generating unit has to be taken off the utility grid. The net result is less power being available to meet consumer demand.

We can thank the Clean Air Act for effectively reducing the amount of airborne pollutants, but we must acknowledge the cost to do so. Electric utilities are for-profit corporations, not charities, and someone has to pay for the increased coal consumption, higher transportation costs, equipment additions, and operating problems that are a result of the usage of low sulfur coal. That someone is the consumer.